The x-ray tube has become essential in medical diagnostic imaging, medical therapy, and various medical testing and material analysis industries. One type of x-ray tube is a computerized tomography (CT) x-ray tube which is used in CT scanners. A typical CT scanner includes a stationary patient receiving region with a gantry mounted for rotation around the patient receiving region. An x-ray tube assembly which produces a radiation beam through an x-ray port across the patient receiving region is mounted to the gantry for purposes of rotation. A coolant fluid is circulated between the x-ray tube assembly and a cooling system (including a heat exchanger and pump) which is also mounted on the gantry. The coolant fluid flows through the x-ray tube assembly to remove heat created during x-ray generation. Finally, an arc or ring of radiation detectors surround the patient receiving region.
During operation, typically, the x-ray tube assembly generates a planar beam of radiation which is then rotated around the body. Various detectors, located around the patient, detect the intensity of the beam. The detectors are connected to a computer which, based on intensity readings, generates an image of a slice of the body. The patient is then moved longitudinally through the gantry with the x-ray tube assembly generating slices so that the computer can generate a three-dimensional image of the body.
In the course of generating slices, much heat is generated by the x-ray tube assembly and this heat must be removed if the service life of the x-ray tube is not to be unduly reduced. As described above, it is known to cool x-ray tubes by circulating a fluid, typically oil, within the tube and externally through a cooling system to remove as much heat as possible. In addition to being used as vehicle for cooling, the fluid is also used for its dielectric properties, in order to insulate the anode connection from ground (and/or the cathode connection), or, depending on the tube assembly, the dielectric oil used in x-ray tubes provides electrical insulation between the high voltage surfaces of the insert and the tube housing which is at ground potential. High voltage arcing can occur between these surfaces if the dielectric constant of the oil is reduced.
Coolant fluid, due to continuous heat and repeated arcing, will eventually break down. When the oil breaks down its dielectric properties as well as its ability to carry away heat (i.e. viscosity) are adversely affected. This results in less electrical insulation where needed, which leads to more arcing and, eventually, tube failure. Hence, proper electrical insulation (i.e., maintaining the proper dielectric property of the coolant fluid) is an important concern in x-ray tube use.
Several causes can reduce the dielectric constant of the oil, such as particulate contaminates, or dissolved hydrogen gas in the oil.
It would be desirable then to have a means for increasing the life of the dielectric oil used in x-ray tubes.